The most important parts requiring lubrication in an internal combustion engine are the three moving valve parts comprising the space between the piston and the cylinder, the bearings and other such bearing parts and the cam and tappet. Of these, the moving valve mechanism which opens and closes the intake valve and the exhaust valve in accordance with the timing of the combustion is an important part which governs the motive efficiency of the internal combustion engine, and it is well known that even when the internal combustion engine is lubricated, the lubrication conditions for this part are very exacting. The prevention of wear and seizure (scuffing) of this part is very important for the long-term retention of the motive efficiency and the reliability of the internal combustion engine. Consequently, wear resistance with respect to the moving valve parts is an important indispensable requirement for lubricating oils for internal combustion engines, and has therefore been included in domestic standard tests for appraising the quality and performance of lubricating oils for internal combustion engines.
Organometallic phosphorus compounds such as zinc dialkyldithiophosphates (ZnDTP) are added to lubricating oils for internal combustion engines as anti-wear agents. However, it has long been feared that these phosphorus compounds adversely affect the performance and lifetime of the catalysts which decontaminate the exhaust gas, and so they tend to be added to the lubricating oil at limited concentrations.
There has been considerable research into using lubricating oils to decrease friction loss and improve fuel cost-efficiency in internal combustion engines. A known method for decreasing viscosity resistance is to lower the viscosity of the lubricating oil for the internal combustion engine. This method decreases engine friction loss and lowers the viscosity of the lubricating oil.
Internal combustion engines which use a piston and cylinder, have a further problem in that some of the combustion gas is blown from between the piston and the cylinder during the combustion process, and leaks into the crank case as blow-by gas. It is known that the nitrogen oxides (NO.sub.x) contained in this blow-by gas cause deterioration of the above-mentioned anti-wear agent ZnDTP, and although adequate anti-wear properties are retained, it is difficult to keep the amount of phosphorus compounds added to a low level.
It is very difficult to maintain the anti-wear properties of lubricating oil for internal combustion engines, particularly when the viscosity and the sulphur content of the lubricating oil for internal combustion engines must be kept low and when, in practice, blow-by gas is present in the engine crank case when engine oil is used. Consequently, recent increases in engine output have tended to result in an increase in the wear and scuffing of all internal combustion engine parts, particularly the moving valve parts such as the cam and tappet, which are subjected to exacting lubrication conditions.
Japanese Unexamined Patent Application No. H5-279686 suggests a lubricating oil composition for internal combustion engines comprising the following indispensable components in the lubricating oil base oil: (a) a molybdenum-based wear-reducing agent chosen from the group consisting of oxymolybdenum dithiocarbamate sulphide (MoDTC) and oxymolybdenumorganophosphorodithioate sulphide (MoDTP); (b) a friction modifier comprising fatty acid ester and/or organic amide; (c) a metallic detergent chosen from the group consisting of calcium sulphonate, magnesium sulphonate, calcium phenate and magnesium phenate; and (d) an ash-free detergent chosen from the group consisting of benzylamine, boron derivatives of benzylamine, alkenyl succinimides and boron derivatives of alkenyl succinimides. This invention aims to achieve good anti-wear properties and a low coefficient of friction, but does not go as far as to consider measures against the NO.sub.x contained in the blow-by gas.
The teaching of Japanese Unexamined Patent Application No. H7-150169 relates to a lubricating oil composition for internal combustion engines comprising the following indispensable components in the lubricating oil base oil: (A) a wear-reducing agent chosen from the group consisting of tungsten salts and molybdenum salts of dithioxanthogenic acid; (B) a friction modifier chosen from the group consisting of fatty acid esters and/or organic amides; and, if necessary, (C) (a) a metallic detergent chosen from the group consisting of calcium sulphonate, magnesium sulphonate, calcium phenate, magnesium phenate, calcium salicylate and magnesium salicylate, (b) an ash-free detergent chosen from the group consisting of benzylamine, boron derivatives of benzylamine, alkenyl succinimides and boron derivatives of alkenyl succinimides, and (c) an anti-wear agent chosen from the group consisting of zinc dithiophosphate (ZnDTP) and zinc dithiocarbamate (ZnDTC). This invention involves the indispensable use of tungsten salts or molybdenum salts of dithioxanthogenic acid, but it too does not go as far as to consider measures against the NO.sub.x contained in blow-by gas.